The types of steel oil rings for internal combustion engines are categorized into a conventional three-piece type oil ring composed of two rings each having a rectangular cross sectional shape and one spring, and a two-piece type oil ring composed of one section wire ring having a groove and one spring.
Recently there has been a desire to change the three-piece type oil ring to the two-piece type oil ring in order to reduce the cost by decreasing the number of elements. A section wire ring having a groove to form the two-piece oil ring has a complicated cross sectional shape, for example, a substantially H- or X-shape. Further, the ratio of the thickness of a web portion connecting the two lateral end flange portions with respect to the overall thickness of each of the lateral end flange portions is very small, resulting in a considerably great degree of deformation.
Further, the material of a piston ring has been needed to be highly alloyed for improving wear resistance, scuff resistance and sulfate corrosion resistance and the like.
Since a pressure ring among the piston rings has a rectangle-like simple cross sectional shape, it does not encounter a critical manufacturing problem. Therefore, high-alloying to 17Cr and to 20Cr-type martensitic stainless steel has been established.
A side rail of a three-piece-combined oil ring has been made of JIS SUS420J2 (0.35C-13Cr type) or 0.65C13Cr type martensitic stainless steel. Also the side rail of the foregoing type has a rectangle-like simple cross sectional shape. Therefore, a side rail of the foregoing type can be manufactured by a conventional cold rolling method or a cold drawing. The rails for the three-piece oil rings have been disclosed in JP-B2-61-54862 (0.65C-13Cr type) and JP-A-61-59066 (0.55C-7Cr type).
Also the oil ring needs to be made of highly alloyed material, and the wear resistance and corrosion resistance can be expected if the two-piece type oil ring is made of highly alloyed material.
However, it is very difficult for a section wire ring having a groove for use in the two-piece type oil ring to be made of martensitic stainless steel containing carbon at a higher ratio as compared with the conventional material, for example, containing, carbon by 0.8 wt% or more and chromium by 15 wt% (hereinafter wt% will be abbreviated to "%") or more. The reason for this is that a ring of the foregoing type has a complicated cross sectional shape and deformed by an excessively higher degree. That is, the cross sectional shape is excessively different from the round or rectangular shape, and therefore, a high plastic working reduction is necessary to form the section shape. In particular, it has been experimentally determined that the structure in which the web portion which is strongly compressed vertically in thickness and flange portions which are not compressed vertically are disposed adjacently results boundary portions which can be very easily cracked. Further, the purpose of use of the oil ring necessitates that the surface roughness must be 3S or lower. In addition, the surface condition is severely restricted to be free from a dent and abrasion. The conventional cold drawing method encounters deterioration in the workability due to high-alloying. Further, the excessively large degree of deformation causes the working cracks to easily take place at boundaries between the web portion and the flange portions as described above. In order to prevent the foregoing problems, the working ratio per one pass or per one annealing process is limited excessively. As a result, the number of passes and annealing processes increase the obtain the final product, resulting in that the surface condition can easily be damaged during the foregoing process or at the time of handling the ring. What is worse, a problem of high cost arises. Hence, a two-piece type oil ring made of martensitic stainless steel containing more than 0.8% carbon and not less than 15% chromium has not been produced.
In order to overcome the foregoing problems, it has been determined that a section wire having a complicated cross sectional shape for use to make an oil ring that has characteristics, such as corrosion resistance and wear resistance, that are required for the oil ring to possess. Further, in Japanese Patent Application Serial No. 3-19302 (1991), a method of manufacturing the section wire for making the steel oil ring is characterized in that a roll die for warm working is proposed. The material disclosed as described above is martensitic stainless steel containing carbon in a quantity more than 0.8% and less than 0.95%, and 15% to 20% chromium and having a complicated lateral cross sectional shape, the foregoing martensitic stainless steel being enabled to be manufactured by the foregoing disclosed method.
The section wire for the two-piece type oil ring must be formed to have a predetermined cross sectional shape and have a plurality of oil passage holes formed in the web portion in a straight array at any step in the manufacturing process. The method of piercing the holes is substantially limited to a punching method in terms of reducing the working cost and improving the working efficiency. However, it has been found that it is very difficult to pierce the foregoing section wire because the material to be worked has been highly alloyed.
Although the oil passage holes can be, by punching, formed in the foregoing low alloy steel even if the material has been quenched and tempered to predetermined hardness of HV300 to 450, the highly-alloyed steel composed as described above cannot be substantially punched by the conventional manufacturing method even if it has been heated to have similar hardness. Further, punching, that can be performed after the conventional low-alloy material has been machined to have a predetermined cross sectional shape, cannot be adapted to the foregoing highly-alloyed material.
Further, it has been found that forming, into a predetermined cross sectional shape, high carbon and high chromium wire material having a substantially the same or substantially the same chemical composition (Cr: 15.0 to 20.0) as that of the material disclosed in Japanese Patent Application Serial No. 3-19302 and subjecting it to continuous annealing under tension enables wire material for an oil ring having a deformed cross sectional shape that exhibits excellent straightness to be obtained. Further, the process for limiting the obtained material to predetermined dimension enables the oil passage holes to be formed by punching. The foregoing technology was disclosed in JP-A-4-333545 (1992). The foregoing dimensional limit sometimes restricts the width b (see FIG. 1) of the pierced oil hole, that is, the cross sectional area of the oil hole, that is, the oil scraping performance. There have been examined to use of low grade fuel, introduce combustion gas into the engine to clear an exhaust gas regulation, use alcohol-type fuel and that of natural gas type fuel in additional to the improvement in the performance of engines, resulting in that a portion of the foregoing technologies has been subjected to actual running tests. There arises a desire of providing, for the foregoing combustion engines, an oil ring exhibiting excellent seizure resistance and corrosion resistance superior to those of the conventional material because the oil deterioration speed is increased and the lubricating ability deteriorates due to a fact that a lack of a supply of oil occurs due to a high speed engine speed, a proportion of water generated after combustion is increased, corrosion is caused to occur easily by sulfur oxides, and corrosion easily takes place due to the presence of carboxylic acid. Recently, there is a trend of using the two-piece type oil ring in a small-size internal combustion engine. Another tendency arises in that the cross sectional area of the oil ring for use in a large-size engine is decreased to reduce the weight of the engine and to reduce the weight of the piston ring. There is another technology about the cross sectional shape of the oil ring that the mutual distance between the flanges, which are brought into contact with the inner surface of the cylinder, is shortened.
Since the foregoing dimensional limitation employed in the foregoing disclosure results in a decrease of the thickness of the piercing punch in proportion to the widthwise dimension of the recessed portion of the wire for the ring, the piercing punch can easily be broken and the oil discharge performance easily deteriorates.